Taxation Without Representation is the motto that is currently on Washington, D.C.'s license plates. It refers to the fact that DC pays federal taxes without getting a federal representative.

"So?", I can hear you saying. Well, here's the thing: based on all other parts of the United States, it should be either both or neither -- not just one. All fifty states both pay federal taxes and get two senators and a numer of representatives proportional to their population. The territories owned by the US (American Samoa, Guam, U.S. Virgin Islands, Puerto Rico, etc) are not represented in Congress, but neither do they have to pay federal taxes. DC is unique in that it is the only territory that pays taxes but has no representative in congress1

Advocates for DC rights seized upon this motto and stuck it on DC's license plates in 2000, obviously alluding to the American Revolutionary meanings mentioned above. While license plates are prohibited from carrying "political messages", it was determined that this was not a political message, but rather a simple statement of fact. Some had wanted to say "No Taxation Without Representation", but that was deemed too political. President Clinton, during the waning days of his office, had his limo's plates replaced with the new plates. President Bush (that's "dubyah" Bush), an opponent of the DC state's rights movement, replaced them with blank DC plates.

Obviously, a license plate isn't going to change anyone's mind; rather, it's more of an attempt to have the problem be noticed by people who wouldn't have otherwise heard of it. Whenever a DC car goes on a road trip, other people may notice its license plate and be moved to try and figure out why DC would choose such an odd slogan. In essence, every car with DC plates has gotten a bumper sticker (albeit in license plate form) that conveys its message wherever it goes. A truly subversive and devious campaign.

1: OK, you got me. While DC has no representative in the Senate, they do have a "shadow representative", who can't vote, but may participate in debates. She (Eleanor Holmes Norton) has little influence in the House.